The invention relates to a diffuser and, more particularly, to a diffuser for a gas turbine engine which delivers the flow from the impeller of a centrifugal compressor to the entrance of a combustor with significantly reduced pressure losses.
Centrifugal compressors generally include a rotating impeller arranged to accelerate and thereby increase the kinetic energy of the gas flowing therethrough. The diffuser is generally characterized by an annular space surrounding the impeller. The diffuser acts to decrease the velocity of fluid flow leaving the impeller and thereby increase its static pressure. Such diffusers may be of the vane or pipe variety. Both of these types have generally included a plurality of circumferentially spaced passages which tangentially converge to the annular space surrounding the impeller. These passages expand in area downstream of the impeller in order to diffuse the flow exiting the impeller. Diffusers of the vane type use a plurality of circumferentially spaced vane elements to form these passages such as disclosed in U.S. Pat. Nos. 3,460,748 issued to J. R. Erwin on Apr. 12, 1969 and 3,369,737 issued to J. R. Switzer et al. on Feb. 20, 1968, both assigned to the assignee of this invention. Alternatively, these passages may be formed as pipe elements between opposed plates as disclosed in U.S. Pat. No. 3,719,430 issued to L. W. Blair et al. on Mar. 6, 1973 and assigned to the assignee of this invention. Both vane and pipe type diffusers generally include a transition region downstream of the diffuser passages to match the diffuser flowpath to the combustor geometry. The transition region generally includes a deswirl section upstream of the entrance to the combustor to remove the circumferential swirl from the flow exiting the diffuser passages. In the diffuser of the Blair et al. patent this structure is characterized by a plurality of circumferentially spaced diffuser passages which gradually merge and exit into a transition region comprising an annular manifold. The manifold includes a plurality of circumferentially spaced apart deswirl vanes which act to cancel the residual circumferential velocity component of the flow exiting the diffuser passages. The flow exiting the deswirl vanes is directed to a combustion chamber.
While demonstrating significant improvements over the performance of prior centrifugal compressors for gas turbine engines, there remains room for improvement in the performance of the diffusers heretofore mentioned, particularly in the area of system pressure losses. In such prior art diffusers the flow exiting the diffuser passages remains at a relatively high velocity when it is deswirled by the deswirl vanes. Deswirl at such high Mach numbers causes a significant reduction in pressure in the flow entering the combustor. Further, flow at such high Mach numbers requires passages with small annulus heights and relatively high tolerances to be maintained in the manufacture of the transition region and the deswirl vanes disposed therein resulting in increased complexity and manufacturing costs for diffuser of the type disclosed in the Blair et al patent and other prior art diffusers which deswirl at relatively high Mach number.